This invention relates to fabrication of a fiber optic coupler and use in a wavelength division multiplexer.
In 1981, Kawasaki, Hill and Lamont discussed a xe2x80x9cBiconical-taper single mode fiber couplerxe2x80x9d (Optics Letters, vol. 6, 1981, p. 327) and the possible use of this device as a beam splitter. Several workers have subsequently examined, and improved upon, a fused biconical taper coupler (FBTC) that fuses two fiber optic lines and provides optical coupling between an input fiber line and one or more output fiber lines.
Bilodeau, Hill, Johnson and Faucher (Optics Letters, vol. 12, 1987, pp. 634-636) have discussed the characteristics of the xe2x80x9cpull signaturexe2x80x9d of an FBTC: as the fused region of two side-by-side fiber optic lines is elongated, each of the amount of signal power that passes along the input fiber and the amount of signal power that passes along the second fiber oscillates with decreasing period, and the sum of these two power values is close to, but less than 1.0 at any elongation value. A small fraction of signal power is lost and does not appear in either fiber. When light having a wavelength xcex in a selected range is launched into the input fiber, the coupling coefficient, representing fractional power in the second fiber, oscillates approximately periodically as xcex varies in this range. This wavelength dependence of coupling coefficient in a limited range of wavelength has been proposed as the basis of a wavelength division multiplexer (WDM), wherein each of the two fibers will carry strong signals with different wavelengths.
McLandrich, Orazi and Marlin (Jour. Lightwave Technology, vol. 9, 1991, pp. 442-447) have proposed a polarization-independent, narrow channel WDM fiber coupler that might operate at a signal wavelength xcex=1.55 xcexcm. The coupler includes two single mode fibers, fused along a selected length, then elongated and tapered so that the cross-sectional shape of the two fibers varies from two approximate circles to one approximate ellipse. Because the signal propagation parameters are not the same for the two transverse polarization directions, this coupler manifests birefringence, and the fraction of signal power coupled into the second fiber will not be the same for the two polarizations.
Wilkinson and Rowe (Electronics Letters, vol. 26, pp. 382-384) have discussed the possibility of applying a twist to a fused assembly of fiber optic lines, for control of signal polarization dependence, but few experimental details are given.
Some of the WDM fiber configurations considered by different workers may be unstable against changing mechanical stress and/or in high temperature environments.
What is needed is a fiber optic system, and method for fabrication, that provides a controllable amount of signal power in each of two or more fiber lines, that is reasonably stable against applied mechanical stress and elevated temperatures, that is substantially independent of signal polarization, that has relatively low power loss, that provides signal wavelength discrimination in each output fiber line and that meets the ITU standards for wavelength division multiplexing.
These needs are met by the invention, which provides a method for fabrication of the desired system. Two or more fiber optic lines, one being a signal input and output line and a second being an auxiliary output line, are positioned contiguous and parallel to each other along an alignment axis over a selected section of each line. Within the selected section: (1) the fiber lines are alternatively heated to a selected temperature T1 and elongated by a selected fraction f of the original length within certain time intervals having selected temporal lengths and measured in order to evaluate compliance with certain ITU standards for wavelength discrimination in a fiber optic coupler; (2) the fiber line temperature is reduced to a second temperature T2 within a second time interval having a selected temporal length; and (3) the fiber lines are twisted about each other by a selected rotational angle within a third time interval having a selected temporal length. The measured optical performance of the resulting fiber optic coupler is brought into compliance with the ITU standards for wavelength discrimination, and the wavelength isolation and polarization independence of the coupler are improved. The resulting fiber optic coupler is allowed to come to room temperature over another time interval.
The resulting fiber optic coupler provides N signal input lines (Nxe2x89xa72), with one input line being used at any one time, and provides N signal output lines. With the choice N=2, by choice of the selected fraction f of fiber optic line elongation, the fraction of signal power in each of the two output lines can be controlled; for example, about 50 percent power in each output line, or a selected power split, such as 10/90, in the two output lines. Choice of the fraction f also determines a sequence of wavelengths, within each output line and possibly differing for each fiber optic line, that are propagated with little or no optical loss in that line. By suitable choices of the fabrication parameters, the channel isolation of the fiber optic coupler is improved, the signal output is made less dependent upon the polarization of the input signal, and the coupler can be made reasonable stable against mechanical stress and against temperature variations.